Elizabeth Bennet speaks her mind ...

,,, maybe that's why 21st century readers like her best.

I've finally figured it out -  though it's highly (highly!) likely that some Austen scholar has posited this concept before. Is there any other Austen heroine who tells someone off, says what we are thinking?
But Elizabeth Bennet tells Darcy what to do w/his lame proposal. She calls him ungentlemanly, rude, severe, unkind - really all the things he was. How perfect. She may have been misled, but she says what we are all thinking - how can you propose to me and insult me at the same time?  You are a jerk. What other Austen character does that? Um, nobody.
S&S: Elinor Dashwood never tells Lucy Steele to go bugger off though she rightfully should. We would after all, wouldn't we?
PER: Anne Elliot never tells Lady Russell thanks for ruining my life and leading me away from the man I did, and still do, love. BWT, he's now throwing himself away on Louisa Musgrove (weak).
MP: Fanny Price, um mouse, doesn't tell Edmund he's making a huge mistake. Or at the least tell Henry Crawford to bugger the frig off because you've got his number.
Emma: Tell Augusta Elton she's crass, base, tacky, and quite frankly really sad for marrying Mr. E.  
NA: Catherine - you have more potential than you realize.. You know Isabella Thorpe is wrong and even hint to her that she is, plus you tell her brother almost to stuff it. You're young, but have the opportunity to make progress (w/ the help of Tilney).
No one but Lizzie actually does it - damn skippy - that's why we like her.